Devices and/or methods in accordance with some embodiments may be employed, for example, in satellite navigation receivers, or their associated front ends, which have an asynchronous interface that does not enable bidirectional transmission due to the limited transmission capacity.
Originally, satellite receivers were employed for position sensing and navigation in the military field, for example for weapon systems, warships and airplanes. Nowadays, however, satellite receivers are increasingly used for non-military purposes as well, for example for seafaring, aviation, by means of navigation systems in cars, for orientation in recreation, in metrology and surveying, in agriculture, in high-performance sports and for use in mobile phones.
For example, the levels of accuracy of satellite receivers nowadays even enable automatic touchdowns in civil aviation, provided that the center lines of the runway were precisely measured beforehand. Another widespread field of use is fleet management on the part of traffic enterprises as well as transport services on land and on water or at sea. For example, if the vehicles or ships are equipped with satellite receivers, the control room stays on top of the locations of the vehicles or ships at any time and can immediately intervene in the case of disturbances. Satellite receivers are also employed, for example, in cars so as to show the driver the way to the desired destination using comprehensive map and city map software, for example by providing the driver with acoustic directional instructions. Recently, the use of satellite receivers on PDA systems (personal digital assistance systems), PNA systems (personal navigation assistance systems) and in mobile phones has seen a sharp increase. Said devices may be flexibly and quickly employed in various vehicles, for example. Mostly, routing is graphically represented on a color screen and with interactive input.
The spreading of satellite receiving terminals may also be explained, in particular, with regard to the constant decrease in prices of electronic products. While fixedly installed systems are typically considerably more expensive than mobile terminals, they have the advantage, however, that they may be coupled to the vehicle's electronics. Thus, additional data which is available in the vehicle and also takes velocity and acceleration into account in order to determine the position more accurately and to be able to determine a position even in areas with no reception, e.g. tunnels, may be rendered usable.
Within buildings, satellite reception is mostly reduced or impossible. In the respective case, this will depend, e.g., on the building materials used in the building and/or their damping properties, and on the location within the building. For example, in the vicinity of a window or in rooms with large windows and an unobstructed sky view, position determination may still be quite possible with reduced accuracy, depending on the current satellite position. In internal spaces, for example cellars, satellite reception is highly restricted, however. Relatively recent satellite receivers enable ensuring reception of the satellite signal also in some situations such as inside buildings, for example. This may be enabled, for example, in that the receive signals are not measured chronologically, and in that not only one receive path is used, but in that a plurality of parallelized satellite receivers are used. By means of such parallel utilization of satellite receivers with simultaneous evaluation, e.g. multipath reception may be highly reduced, for example, so that in combination with an increased input sensitivity of the satellite signal, also those signals that have been reflected by walls and floors may possibly still be evaluated inside buildings or in narrow alleyways in densely built-up areas.
For example, satellite receiving systems measure the receiver's position by means of the distance from several satellites. For example, the satellites continuously emit their changing positions and the precise time of day. From their signal propagation times, the satellite receivers may then calculate their own positions and velocities. For this purpose, for example, three satellites may be used for determining the space coordinates, and a fourth satellite may be used for determining the time coordinate. By means of the satellite signals, not only the position, but also the velocity of the receiver may be determined, which may be effected, for example, by measuring the Doppler effect.
For example, satellites emit “spread spectrum” modulated signals. A data signal may be modulated, for example, with a pseudo-random code sequence and be received by the receiver by means of cross correlation. For particularly efficient transmission, code sequences, for example, may be used which exhibit a specific code phase shift. For example, the satellite signals are emitted, by means of the specific coding, such that the resulting transmission sequences of various satellites are orthogonal to one another, so that independent reception of the individual satellite signals becomes possible even though all of the satellites emit at the same frequencies. This code division multiple access (CDMA) is used in most satellite receiving terminals for evaluating the transmit signal(s), for example. For example, Gold sequences may be used for this which may be generated, for example, from two generator polynomials by means of fed-back shift registers, it being possible to use a code phase shift between the two generators in order to achieve that different Gold sequences having the identical generator polynomials are positioned within the code space such that they are almost orthogonal to one another and therefore hardly influence one another. In this respect, the received satellite signals have a sufficiently small cross correlation for the CDMA reception due to the code phase shifts, so that a plurality of satellites of the same transmit frequency may transmit data to the satellite receivers.
To increase accuracy, the signals may also be sent out by a satellite at several frequencies, for example; in most cases, one may select which code may be transmitted at which frequency. Due to the transmission at several frequencies, ionospheric effects, for example, which lead to an increase in the propagation time, may be subtracted out so as to increase the level of accuracy.
For example, a typical satellite receiver may operate in accordance with the principle that, for a received signal of a satellite, a Gold code sequence is generated which corresponds to the Gold code sequence emitted by the satellite. The code sequence received at, and the code sequence generated within the receiver itself initially are not temporally related. To establish this temporal relationship, both sequences are multiplied by each other after one of the sequences has undergone a temporal shift, and the multiplication results are added. This procedure may also be referred to as cross correlation. If the temporal shift is varied, the sum will change. For example, the sum will be at a maximum if the sequences temporally coincide.
For example, it is also possible—due to the specific code sequences emitted by the satellite—to ensure that the maximum of the cross correlation occurs only at the correct code sequence and at the correct temporal shift, which may also be referred to as uniqueness. By counting within the signal units, and by means of positional evaluation of the current time within the code block, it is possible, for example, to determine the precise transmission time when the received signal was emitted by the satellite. For the evaluation it is sufficient, for example, for only the time of the beginning of a code block within the satellite to be known. The receiver may then measure the time between the evaluation time and the beginning of a code block to determine the transmission time of the code block by evaluating the satellite message.
Because of the signal evaluation by means of cross correlation determination and evaluating the maximum of the cross correlation, it is particularly important that the temporal coherence of the receive signal is maintained. For example, a cross correlation can be correctly evaluated only if both signals over which the cross correlation is performed have a correct temporal relationship to each other. This means that a receive signal should have the same temporal reference as a transmitted signal so as then to be able to determine, by means of correlation, the temporal shift as compared to the transmitted signal.
The satellite navigation receiver mostly has only limited transmission capacity available to it for forwarding the signals received from the satellites to a checkpoint, for example, said limited transmission capacity enabling no bidirectional transmission, for example. Applications in this context comprise forwarding, with a low protocol overhead, the received data of a satellite navigation terminal to, e.g., a checkpoint which may enable, for example by means of the data of several satellite receive terminals, checking and monitoring the positions of the individual satellite navigation receivers. In this manner, for example, effective fleet management on the part of, e.g., traffic enterprises as well as efficient control of, e.g., transport services on land, in the air or at sea may be enabled. By means of the data sent by the satellite receivers, the control room stays on top of the locations of the various vehicles or ships or flying objects at any time and can immediately intervene in the case of disturbances.
For example, the satellite receiver which, in this communication, becomes the transmitter so as to transmit the data received from the satellites to the checkpoint, has an asynchronous interface available to it. The data stream is emitted by the transmitter, for example, as soon as the data is available to the transmitter and without the transmitter paying attention to the receiver. This means that due to the lacking reverse channel, no “handshake” can occur between the transmitter and the receiver. Since the transmitter receives no feedback from the receiver, any errors that occur on account of the transmission link cannot be balanced off by re-transmitting the defective packets. The data stream may consist, e.g., of a succession of packets that the transmitter may transmit to the receiver via its asynchronous interface with a low protocol overhead.